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Frost heaving (or frost heave) is the process by which the freezing of water-saturated soil causes the deformation and upward thrust of the ground surface. This process can damage plant roots through breaking or desiccation, cause cracks in pavement, and damage the foundation of buildings, even below the frost line. Moist, fine-grained soil at certain temperatures is most susceptible to frost heaving.

Frost creep, an effect of frost heave, involves a freeze-thaw action allowing mass movement down slope. The soil or sediment is frozen and in the process moved upward perpendicular to the slope. When thaw occurs the sediment moves downwards thus mass movement occurs.

Mechanisms

Molar volume expansion

The earliest known documentation of frost heaving came in the 1600s. The molar volume of water expands by about 9% as it transforms from water to ice at its bulk freezing point. Originally, frost heaving was thought to occur due simply to the freezing of water that was present in the soil prior to the onset of subzero temperatures, and which froze in place without moving. If this were the sole source of expansion, 9% would be the maximum expansion possible, and even then only if the ice were rigidly constrained laterally in the soil so that the entire volume expansion had to be taken up vertically. However, the vertical displacement of soil in frost heaving can be significantly greater than that due to molar volume expansion. Ice is unusual in that there is an increase in molar volume when freezing. Most compounds show contraction on transforming from liquid to solid. Classic expermients by Taber first demonstrated a flow of liquid water towards a cold front, and that liquids such as benzene, which contracts when it freezes, also produces frost heave, thereby eliminating molar volume changes as the only mechanism for vertical displacement. These experiments also demonstrated the generation of ice lenses inside columns of soil that were frozen by cooling the upper surface only, thereby establishing a temperature gradient. Therefore the molar volume expansion of water cannot be the sole, and may not be a major, contributor to frost heave.

Liquid water source, transport, and existence below the bulk freezing point

As the heaving may be greater than that possible due to the 9% expansion of water on freezing, this requires more water to flow into the freezing region, which in turn requires a source of liquid water and a means of transport. During frost heave, one or more soil-free ice lenses grow, and their growth displaces the soil above them. One source of water is from water at depth, where the temperature is above the bulk freezing point. However, at the ice lens, the temperature is obviously at or below the bulk freezing point. However, this does not shut off the water supply, as liquid water can exist below its bulk freezing point. One effect that allows for liquid water to exist below the bulk freezing point is the Gibbs-Thomson effect of confinement of liquids in pores. Very fine pores have a very high curvature, and this results in the liquid phase being the thermodynamically stable phase in such media at temperatures sometimes several tens of degrees below the bulk freezing point. Flow of liquid water through fine pores would be one mechanism to supply growing ice lenses in soils. Another effect is the preservation of a few atomic layers of liquid water on the surface of ice, and between ice and soil particles. This unfrozen layer of water is also known as premelted water and has been known to exist since the nineteenth century. Ice premelts against its own vapour, and in contact with silica.

Thermal regelation

The same intermolecular forces that cause premelting at surfaces have been shown to cause heaving. If ice surrounds a fine soil particle against which it premelts, the soil particle will be displaced in the direction of the thermal gradient due to melting and refreezing of the thin film of water that surrounds the particle. The thickness of such a film is temperature dependent and is thinner on the colder side of the particle. Water has a lower free energy when in bulk ice than when in the supercooled liquid state. Therefore, there is a continuous replenishment of water on the cold side, by flow of water from the warm side to the cold side, and continuous melting to re-establish the thicker film on the warm side. The particle is forced towards the warm direction. This process is called thermal regelation The ice repels foreign particles, and a 10 nanometer film of unfrozen water around each particle can lift a micron-sized particle by 10 microns/day in a thermal gradient of as low as 1 Km-1. Therefore the ice lens can purge itself of any particles that are entrained, and tends to reject them at its interface in the first place. The ice lens can both lift the soil above it and itself, by pushing particles downwards towards its lower (warmer) interface, which clearly must remain at or below the bulk freezing temperature. If the air temperature is below freezing but relatively stable, the heat of fusion from the water that freezes can cause the temperature gradient in the soil to remain constant.

As the liquid water freezes onto the ice lens, soils draw in further liquid water from the network of unfrozen films that exist on the scale of a few nm in the soils around them. In doing so the free energy of the whole system is lowered.

Susceptible soil types

Frost heave relies on soils in which there is a supply of liquid water to feed growing ice lenses, established in a thermal gradient, that are capable of displacing the soil perpendicular to that gradient. This requires:
  • freezing temperatures
  • a supply of water
  • a soil that has:
    • the ability to conduct water
    • a high affinity for water
    • saturation (i.e. the pore spaces are filled with water)


Silty and loamy soil types are susceptible to frost heaving. The affinity of a soil for water is generally related to the surface area of the particles that it is composed of. Clays have a high ratio of surface area to volume and have a high affinity for water. Larger particles like sand have a lower ratio of surface area to volume and therefore a low affinity for water.

Conversely, the hydraulic conductivity of a soil is related to the pore size. Soils composed of very small particles like clay have small pores and therefore low hydraulic conductivity. Soils composed of larger particles like sand have larger pores and a higher hydraulic conductivity.

The offsetting nature of these two requirements mean that clayey and sandy soils are less conducive to frost heaving than silt, which has a moderate pore size and moisture affinity.

Frost creep: Soil locomotion due to frost heave

Frost creep, an effect of frost heave, involves a freeze-thaw action allowing mass movement down slope. The soil or sediment is frozen and in the process moved upward perpendicular to the slope. When thaw occurs the sediment moves downwards thus mass movement, or locomotion, occurs.

Structures created by frost heaving

In Arctic regions, frost heaving for hundreds of years can create structures, known as pingosmarker, as high as 60 metres. Frost heaving is also responsible for creating stones in unique shapes such as circles, polygons and stripes. A notable example is the remarkably circular stones of the islands of Spitsbergenmarker.

Polygonal forms caused by frost heave have been observed in near-polar regions of Mars by the high-resolution HiRISE camera on the Mars Reconnaissance Orbiter. In May 2008 the Mars Phoenix lander touched down on such a polygonal frost-heave landscape and quickly discovered ice a few centimetres below the surface.

For further information, see: patterned ground.

See also

  • Frost law
  • Lithalsa
  • Pingomarker, also called hydrolaccolith, a mound of earth-covered ice found in the Arctic and subarctic area that can reach up to 70 m in height and up to 600 m in diameter.
  • Palsa, a low oval elevation in areas with permafrost, frequently peat bogs, where a perennial ice lens has developed within the soil.


References



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